GRK2 and chronic pain

Abstract

G protein coupled receptor kinases (GRKs) regulate the termination of G protein-coupled receptor (GPCR) activation by phosphorylating agonist-occupied GPCRs. Apart from terminating GPCR activation, more evidence emerges in recent years demonstrating that GRKs also interact with multiple intracellular signaling molecules, suggesting novel regulatory roles of GRKs. GRK2 has first been identified as a β-adrenergic receptor kinase (βARK). As the most studied member of GRK family, GRK2 is particularly highly expressed in immune and nervous system and is involved in many disease processes. Our previous work indicated that a reduced intracellular GRK2 protein level is related to prolonged inflammatory pain. In this thesis, we further investigated how GRK2 regulates the duration and severity of inflammatory pain. We focussed on: 1. the role of neuronal GRK2 in regulating inflammatory hyperalgesia. 2. the interaction of GRK2 with intracellular pain signaling pathways. 3. the potential to treat chronic pain by upregulating the GRK2 protein level. In chapter 2 and 3, we investigated whether GRK2 modulates inflammatory mechanical and thermal hyperalgesia by using heterozygous GRK2 knockout mice, which express 50% of the GRK2 protein. In these chapters, we used the inflammatory mediator, prostaglandin E2 (PGE2), and the stress hormone epinephrine (EPI), to produce thermal and mechanical hyperalgesia. We show that: 1. Both PGE2- and EPI-induced thermal and mechanical hyperalgesia are prolonged significantly in GRK2+/- mice. 2. GRK2 level in nociceptors is crucial to establish the prolonged PGE2- and EPI-induced hyperalgesia. 3. A reduced level of GRK2 in microglia/macrophages has a modest effect on EPI-induced hyperalgesia. A low GRK2 level in astrocytes has no effect on PGE2- and EPI-induced hyperalgesia. 4. In both PGE2- and EPI-induced hyperalgesia, low nociceptor GRK2 leads to prolonged hyperalgesia via switching signaling from a cAMP-PKA-dependent towards a Epac1-PKCε-dependent pathway. In chapter 4 and 5, we further investigated the role of GRK2 in inflammatory pain, by using a more chronic inflammatory pain model induced by hyperalgesic priming. In these 2 chapters, we show that: 1. A low dose of carrageenan, a PKCε activator (ΨεRACK), and also glial-derived neurotropic factor (GDNF) can produce hyperalgesic priming in mice. 2. The transition to chronic hyperalgesia caused by a prior nociceptive sensitization or priming with carrageenan or ΨεRACK or GDNF is associated with a decrease in nociceptor GRK2 and/or an increase in Epac1 level. 3. Transition to chronic pain that occurs after hyperalgesic priming can be inhibited by upregulating GRK2 or downregulating Epac1 during the primed state.